The present invention generally relates to a flow control that includes a flow sensor, and more specifically, to a micro/miniature flow control in which an electric potential is employed to control fluid flow through the device.
Fluid control in portable and implantable medical devices typically requires techniques be employed that are uniquely suited to micro/miniature fluid circuits. For example, conventional mechanical or electromechanical valves are too large and often too slow to be used in such applications. Other types of fluid valves require more space than is available in micro/miniature fluid circuits. Examples of mechanical valves and some of their characteristics and limitations are: shape memory alloy actuated valves (actuated by changes in temperature, but subject to fatigue failure), thermopneumatically actuated valves (typically electrochemically activatedxe2x80x94may require several minutes to respond and are temperature sensitive), bi-morph (Al/Si) (reliability problems and typically capable of less than 1 mm stroke), Nixe2x80x94Si based valves (thermally activated and typically capable of less than 1 mm stroke), mini-solenoid actuated valves (good reliability and relatively small stroke), and electrostatic valves (very reliable and characterized by short actuation distance). The various types of mechanical valves listed above require an area of at least 4 mmxc3x974 mm, which is much more than is generally available in a micro/miniature fluid circuit. While micromechanical valves are available that are smaller than the conventional mechanical valves discussed above, such valves are typically designed to control gas flow by moving a membrane over an orifice and are generally not suitable for controlling the flow of a liquid.
A more suitable type of valve for micro/miniature fluid circuit applications, because it requires much less area to operate, is sometimes referred to as a xe2x80x9cvirtual valve.xe2x80x9d Traditional valves have moving components that regulate flow. A virtual valve has the same characteristics of a mechanical valve, except that there are no moving parts in a virtual valve. Virtual valves take advantage of microfluidic characteristics such as surface tension or pressure gradients to regulate fluid flow. Some virtual valves employ an externally applied pressure to move fluid. Pressure balanced virtual valves may also employ external pneumatic pressure or convert kinetic energy to pressure, but tend to be dependent upon channel shape. Bubble valves are another type of virtual valve that are designed to generate bubbles to block fluid flow by creating temperature gradients. Pressure balanced virtual valves serve the function of dual check valves in pumping circuits and may comprise pairs of tapered channels (with the tapers directed in opposite directions) that tend to permit fluid flow in one direction, but not the other. Although pressure balanced valves have an advantage because they do not require moving parts, they are not leak free, and fluid flow is usually not symmetric through the pairs of tapered channels.
It may be necessary to monitor fluid flow in a microfluid circuit. Often, due to the small size of the passages in such devices, the rate of fluid flow is too low to be measured by conventional flow sensors. For example, a thermal flow sensor does not have sufficient sensitivity to monitor flow rates less than 1.0 ml/hr. In some applications, the rate of flow is measured in xcexcl/hr., which is far below the range of mechanical flow sensors. The typical full-scale range for a micro/miniature flow sensor is three orders of magnitude higher than the required accuracy. Most flow sensors currently used for such applications are of the thermal sensor type in which the temperature is measured around a heated element to determine the rate of fluid flow as a function of heat dissipated in the fluid flowing past the element. Another thermal flow measurement technique applies heat pulses to an element disposed in a fluid channel; the phase shift of the first harmonic of the temperature pulses is inversely proportional to the flow velocity of fluid past the element. Pressure based flow sensors apply the Bernoulli principle and use capacitive or resistive elements, drag force sensors, anemometers, acoustic Doppler sensors, and Coriolis sensors. Each type of flow sensor has desirable virtues, but most are not suitable for monitoring low fluid flow in micro/miniature fluid circuits, either because of their lack of sensitivity, slow response time, excessive size, or because they require excessive power.
Bubble sensors are also often required in medical infusion pumps to monitor the quality of liquids being infused into a patient. The techniques typically used for sensing bubbles in a fluid stream detect the bubbles by sensing changes in acoustic signals propagated through the liquid, changes in a permitivity measured across the liquid stream, variations in an optical light path, or changes in the output of a hydrophone sensor. Not all of these techniques are particularly applicable to micro/miniature fluid circuits because of size limitations. For example, the piezoelectric transducers used for generating and receiving sound waves transmitted through a fluid stream are not readily produced in micro/miniature size. Sensing bubbles by their effect on light passing through a fluid stream requires little power and has a fast response time, but may not work well if the liquid is opaque. Hydrophones are generally too large and require too much complexity in the required supporting electronics to be practical for detecting bubbles in micro/miniature fluid circuits. Capacitive bubble sensors are relatively simple, comprising two spaced-apart metal plates disposed on opposite sides of a liquid path in the fluid circuit, for sensing changes in permitivity occurring when a bubble passes between the plates.
Applications for micro-miniature fluid control circuits include medical apparatus, such as implantable liquid metering infusion systems and pump cassettes, for administering drugs and other medicinal fluids. Such fluid control circuits are also usable in gravity fed tube sets for infusing liquids into a patient""s cardiovascular system. The size of portable devices of this type that are self-contained (i.e., not coupled to an external fluid source) is generally a function of the size of the fluid reservoir that is required. For example, an infusion pump the size of a conventional electronic pager will likely have a reservoir of about 5-20 ml. If the pump is the size of a man""s wrist watch, its reservoir will hold about 5 ml. A pump the size of a nickel will have a reservoir holding about 1-2 ml. Implantable pump devices or those introduced orally or by injection through a syringe will be correspondingly smaller and only able to administer substantially smaller quantities of a liquid.
Several techniques can be used to provide a positive actuation for pumping a liquid or for producing other actions involving the application of force in a micro/miniature fluid circuit. These techniques typically rely on either thermal actuation, electrostatic actuation, or magnetic actuation, but tend to have drawbacks because they either require high power (greater than 100 mW), or a relatively high voltage (greater than 30 volts) to operate. Thermal actuation can achieve a phase change in a material such as a shape memory alloy or change the length of a member due to thermal expansion/contraction. Resistive heating can be employed to produce the temperature change. Electrostatic, electrohydrodynamic, or electro-osmosis forces can be generated by applying a voltage differential to materials. For example, if one material is a membrane, a bridging member, or a cantilever, the electrostatic bias will cause the member to move relative to an opposite member to which the bias voltage is applied. In pumps employing electrohydrodynamics, fluid is moved under the influence of an electric field. Up to 1000 volts may be required to energize electrostatic and electrohydrodynamic actuators, and the conductivity of the fluid may preclude the use of electrohydrodynamics.
Piezoelectric actuators offer another possible option, but may be limited by difficulties arising in transferring the technology from ceramics to thin films like those typically used in micro/miniature fluid circuits. Magnetic actuators typically require an electromagnetic coil and may also require a permanent magnet, which can be difficult to form in a micro/miniature fluid circuit.
As will be evident from the above discussion, currently available technology is not well suited for use in fabricating valves, flow sensors, bubble sensors, and actuators in micro/miniature fluid circuits. Accordingly, it will be apparent that a new approach is needed to achieve these functions if such fluid circuits are to be successfully commercially developed for medical and other applications.
In accord with the present invention, a monolithic fluid flow control structure is defined that includes a fluid channel extending through the fluid flow control structure between an inlet port and an outlet port. The inlet port is adapted to couple in fluid communication with a fluid reservoir from which fluid is supplied to the inlet port. A virtual valve is disposed in the fluid channel, upstream of the outlet port. The virtual valve controls fluid flow through the outlet port in response to a valve control signal applied to the virtual valve. A first pressure sensor and a second pressure sensor are included, and at least one is disposed within the fluid channel, between the inlet port and the outlet port. The first and second pressure sensor respectively produce first and second pressure signals that are employed in sensing fluid flow through the fluid flow control structure as a function of the differential pressure.
A bubble sensor is preferably provided and includes a first plate and a second plate disposed on opposite sides of the fluid channel. The first and second plates sense bubbles in a fluid flowing through the fluid channel as a function of a change in capacitance or permitivity between the plates.
Both the first pressure sensor and the second pressure sensor are preferably disposed within the fluid channel. Alternatively, the second pressure sensor is disposed downstream of the outlet port, and the first and the second pressure sensors comprise a differential pressure transducer that senses a differential between a pressure within the fluid channel and a pressure downstream of the outlet port.
The virtual valve includes at least one passage having a transverse cross-sectional dimension that is less than 5 xcexcm. A voltage is applied to the virtual valve to bias or counter bias fluid flow through the virtual valve.
In the monolithic fluid flow control structure, the fluid channel is formed of a biologically inert material, and preferably, in a slab of silicon. In one form of the invention, the monolithic fluid control structure is sufficiently compact in size to be injected into a patient""s body through a hypodermic syringe.
Another aspect of the present invention is directed to a method for controlling and monitoring fluid flow in a micro/miniature device. The steps of the method are generally consistent with the functions implemented by the elements of the monolithic fluid flow control structure discussed above.